Applicability Analysis of Different Diagnostic Methods for Boundary Layer Height in Beijing–Tianjin–Hebei Region and Its Surrounding Areas

Radiosonde data collected from seven stations in the Beijing–Tianjin–Hebei region and its surrounding areas from 2016 to 2021 were analyzed to assess the suitability of five different diagnostic methods for calculating the atmospheric boundary layer height. These include the traditional potential temperature gradient, improved potential temperature gradient, relative humidity gradient, specific humidity gradient, and Richardson number methods. The results show that the difference in the boundary layer height calculated by these five methods is between 40 and 1000 m. The boundary layer height determined using the traditional potential temperature gradient, relative humidity gradient, and specific humidity gradient methods is generally higher with large structural uncertainty. Meanwhile, the Richardson number method and improved potential temperature gradient method yield lower boundary layer heights, with the former presenting the least uncertainty. The boundary layer heights obtained with the five methods at the three representative stations of Laoting, Beijing, and Taiyuan exhibit obvious seasonal and intraseasonal differences. Among them, the two humidity-based methods present the characteristics of high boundary layer height in summer and autumn and low boundary layer height in spring and winter in Laoting. Meanwhile, they show high boundary layer height in spring and summer and low boundary layer height in autumn and winter in Beijing and Taiyuan. The other three methods display high values in spring and summer and low values in autumn and winter for all three stations. The relative humidity gradient method and specific humidity gradient method exhibit greater parameter uncertainty, followed by the traditional potential temperature gradient method and improved potential temperature gradient method. When examining the time consistency of the monthly mean of the boundary layer height calculation results, correlations between the methods are markedly higher at night than those during the day, and the correlation coefficient between the two humidity-based methods is consistently the highest. However, the correlation among the other three methods is poor. The Richardson number method and the improved potential temperature gradient method are highly correlated, and the traditional potential temperature gradient method fluctuates more greatly than the other methods due to differences in stations.